Machines of this kind are disclosed in GB-A-168, 147.
It is common practice to set the grinders close together, but not in mutual contact; a knife to be sharpened is moved through the gap between the grinders such that the latter will restore the knife to a sharp edge by their abrasive action.
In this way, fully satisfactory results cannot be attained, however, because of the knife blade acquiring a cross-sectional shape which substantially duplicates the geometry of the two facing grinders, that is, a concave shape of its sides. This feature is objectionable both because it provides no knife edge of zero thickness (thereby requiring an additional finishing step), and because it weakens the sides of the knife blade. Ultimately, the quality of the sharpening action afforded by such prior machines is much poorer than that to be obtained manually by a skilled knife grinder.